The invention relates to a drag head of a trailing suction hopper dredger according to the preamble of claim 1. The invention relates more particularly to a drag head which comprises a rotatable visor connected to a suction pipe for discharging soil via the suction opening of the visor.
Such a drag head is known, for instance from EP-A-0892116. Described herein is a drag head consisting of a visor connected to a suction pipe. The visor generally has an upper wall and two side walls and is open on the underside, thereby creating a suction opening for the discharge of soil. The whole is fixed to the trailing suction hopper dredger by means of a drag pipe. In order to enable dredging of ground under water, the drag head is lowered with drag pipe and suction pipe under water at the position of the rear side of the trailing suction hopper dredger until it contacts the bottom under the influence of its own weight. The drag head is dragged over the bottom for dredging by the movement of the trailing suction hopper dredger, wherein the soil is loosened and is suctioned away with water via the suction pipe. The trailing suction hopper dredger is provided for this purpose with a suction pump. The loosening of the ground is facilitated in the known drag head by providing the visor with a series of teeth, which are generally arranged transversely of the sailing direction on a so-called toothed beam and which penetrate partially into the ground during the dredging. Water under pressure is also injected into the ground in order to fluidize it. All these measures are intended to increase the dredging efficiency, which in this application is understood to mean the volume of soil dredged per unit of time.
If the known drag head is applied on harder grounds, such as for instance in sandstone, coral, rock or highly compacted mud, large hard chunks can result during dredging due to the action of the teeth, and these are suctioned up in their entirety and may block the suction pipe or even damage or put the suction pump out of operation. Many grounds for dredging are also strewn with armaments, such as bombs. In order to avoid such undesirable objects causing problems, the known drag head, and more particularly the visor thereof, is provided with a catching construction in the form of for instance a grid. Such a framework of rods running crosswise, between which passage openings are situated, prevents undesirable objects which are larger than the passage openings from entering the suction pipe. It has however been found that this has an adverse effect on the dredging efficiency. A decrease in the dredging efficiency is caused, among other factors, by a part of the suction opening of the visor becoming blocked by the undesirable objects, whereby increasingly less soil can be suctioned up. The drag head must then be brought to the surface and cleaned, which takes up valuable time.